HVAC systems incorporating fresh air return and indoor air quality controls create a number of options for air quality control. The use of outdoor air for ventilation creates the possibility of providing thermal (often cooling) control through the use of ambient air, rather than artificially changing temperatures. It is widely believed that, usually, infusion of outdoor air into a controlled space provides health and comfort benefits. However, controls over temperature, fresh air infusion, humidity, and air quality can each present sometimes conflicting aims and needs. Further, as the system becomes more complex, more sensors and devices are used in the HVAC system. With increased numbers of sensors and devices comes a greater likelihood of failure or malfunction, preventing effective implementation of desired strategies that rely on information or operation of sensors or devices that have failed.